Amorphous alloys have generally been prepared by rapid quenching from above the melt temperatures to ambient temperatures. Generally, cooling rates of 105° C./sec have been employed to achieve an amorphous structure. However, at such high cooling rates, the heat can not be extracted from thick sections, and, as such, the thickness of articles made from amorphous alloys has been limited to tens of micrometers in at least in one dimension. This limiting dimension is generally referred to as the critical casting thickness, and can be related by heat-flow calculations to the cooling rate (or critical cooling rate) required to form an amorphous phase.
This critical thickness (or critical cooling rate) can also be used as a measure of the processability of an amorphous alloy. Until the early nineties, the processability of amorphous alloys was quite limited, and amorphous alloys were readily available only in powder form or in very thin foils or strips with critical dimensions of less than 100 micrometers. However, in the early nineties, a new class of amorphous alloys was developed that was based mostly on Zr and Ti alloy systems. It was observed that these families of alloys have much lower critical cooling rates of less than 103° C./sec, and in some cases as low as 10° C./sec. Accordingly, it was possible to form articles having much larger critical casting thicknesses of from about 1.0 mm to as large as about 20 mm. As such, these alloys are readily cast and shaped into three-dimensional objects, and are generally referred to as bulk-solidifying amorphous alloys.
A unique property of bulk solidifying amorphous alloys is that they have a super-cooled liquid region, ΔTsc, which is a relative measure of the stability of the viscous liquid regime. It is defined by the temperature difference between the onset of crystallization, Tx, and the glass transition temperature, Tg. These values can be conveniently determined by using standard calorimetric techniques such as DSC (Differential Scanning Calorimetry) measurements at 20° C./min. For the purposes of this disclosure, Tg, Tsc and Tx are determined from standard DSC scans at 20° C./min. Other heating rates such as 40° C./min, or 10° C./min can also be utilized while the basic physics of this technique are still valid. All the temperature units are in ° C. Generally, a larger ΔTsc is associated with a lower critical cooling rate, though a significant amount of scatter exists at ΔTsc values of more than 40° C. Bulk-solidifying amorphous alloys with a ΔTsc of more than 40° C., and preferably more than 60° C., and still more preferably a ΔTsc of 80° C. and more are very desirable because of the relative ease of fabrication. In the supercooled liquid region the bulk solidifying alloy behaves like a high viscous fluid. The viscosity for bulk solidifying alloys with a wide supercooled liquid region decreases from 1012 Pa s at the glass transition temperature to 107 Pa s. Heating the bulk solidifying alloy beyond the crystallization temperature leads to crystallization and immediate loss of the superior properties of the alloy.
Jewelry accessories made from amorphous platinum alloy have to withstand temperatures up to 200° C. In order to use the alloy for jewelry accessories it has to maintain its amorphous nature up to 200° C. This means that the glass transition temperature should be above 200° C. On the other hand, the glass transition temperature should be low in order to both lower the processing temperature and minimize shrinkage due to thermal expansion.
Another measure of processability is the effect of various factors on the critical cooling rate. For example, the level of impurities in the alloy. The tolerance of chemical composition can have major impact on the critical cooling rate, and, in turn, the ready production of bulk-solidifying amorphous alloys. Amorphous alloys with less sensitivity to such factors are preferred as having higher processability.
In general, Pt-rich bulk amorphous alloys have compositions close to the eutectic compositions. Therefore, the liquidus temperature of the alloy is in generally lower than the average liquidus temperature of the constituents. Bulk solidifying amorphous alloys with a liquidus temperature below 1000° C. or more preferably below 700° C. would be desirable due to the ease of fabrication. Reaction with the mold material, oxidation, and embrittlement would be highly reduced compare to the commercial crystalline Pt-alloys.
Trying to achieve these properties is a challenge in casting commercially used platinum alloys due to their high melting temperatures. For example, conventional Pt-alloys have melting temperatures generally above 1700° C. These high melting temperature causes serious problems in processing. At processing temperatures above the melting temperature the Pt alloy react with most investment materials which leads to contamination, oxidation, and embrittlement of the alloy. To process alloys at these elevated temperatures sophisticated expensive equipment is mandatory. In addition, during cooling to room temperature these materials shrink due to crystallization and thermal expansion. This leads to low quality casting results. In order to increase the properties subsequent processing steps such as annealing are necessary.
Another challenge in processing commercial crystalline Pt-alloys is that during crystallization the alloy changes its composition. This results in a non-uniform composition in at least at portion of the alloy.
Accordingly, a need exists to develop platinum rich highly processable bulk solidifying amorphous alloys. The desired Pt-base amorphous alloys have a low melting and casting temperatures of less than 800° C., a large supercooled liquid region of more than 60° C., a high fluidity above the glass transition temperature, and a high resistance to against embrittlement during processing above around the glass transition temperature.